The present invention relates to an apparatus for the coating of a substrate, in particular a circuit board with a device (material application device) for applying a coating material, in particular a protective lacquer, and with a device for supplying a gaseous medium (gas supply device), the material application device having an inner tubular element, the gas supply device having an outer tubular element which is arranged coaxially to the inner tubular element and surrounds the latter, so as to form, between the outer and the inner tubular element, a gas supply duct which has an annular orifice at one end, the supply duct being configured so that the gaseous medium flows out, parallel to the coating material, through the annular orifice, in order, when it impinges on the substrate, to displace the applied coating material and thereby to distribute it over the area.
An apparatus of the abovementioned type is known from the publication DE 10261576 A1.
In general, apparatuses of this type are used, for example, for providing circuit boards with a protective lacquer. An appreciable problem in the coating of circuit boards with a protective lacquer is, in particular, that of applying the protective lacquer accurately. Minor deviations or splashes of protective lacquer may lead very quickly to a contamination of contacts, this being reflected later in operating faults of the circuit.
In general, spraying methods, as they are known, are often used, which, by means of spraying nozzles of differing shape, atomize lacquer when it leaves a nozzle. Air-assisted atomization of the lacquer gives rise to a lacquer film closed in the middle and becoming depleted outwardly, with a spray mist and splashes thereby being formed. This spraying method is suitable particularly for manual spraying by means of paint spray guns because of the transitions which run away into one another. A broad overlapping of the spray passes is a precondition for a uniform spray pattern, and therefore no sharply delimiting lacquering edge can be formed.
A lacquering pattern with an exact delimitation of the lacquering surface and with less of a spray mist is obtained by means of what are known as airless spray systems. Since these require exact reproducible spraying, the spray valves used are guided by means of robot systems. The overlapping of the spray passes should in this case be as low as possible, in order to avoid accumulations of material. Various spray valves and designs are known in the prior art.
A known method for applying a protective lacquer is known by the name of “Select-Coat® method” and is based on a slotted nozzle. The protective lacquer is pressed through this nozzle. However, the least impurities cause disruption in maintaining the lacquer film. Substrates with higher obstacles, such as are presented, for example, by a printed circuit board module, cannot be coated satisfactorily, because spray shadows are formed behind each passed-over obstacle as a function of the speed and the component height. Splashes also occur under these conditions. The viscosity of the lacquers should not exceed 200 mPas. The formation of splashes otherwise increases.
For higher-viscosity coating materials, there is a method which is called the “Swirl-Coat method” and is based on a spray jet moved circularly or elliptically. This method is well suited to low-populated surfaces in the case of printed circuit board modules, whereas widely differing layer thicknesses occur in the case of surfaces populated with higher components. The application width of the spray surface also changes with the unavoidably necessary change in the distance of the nozzle from the substrate. This method is not suitable for applying low-viscosity substances and lacquers to printed circuit board modules, because excessive differences arise in the lacquer height. The spray head cannot penetrate between high components because of its size.
Furthermore, during lacquering, filaments remain caught on components and interrupt the lacquering pattern, thereby generating splashes. Moreover, marginal regions cannot be coated cleanly by means of this method.
DE 33 29 880 A1 discloses a spray valve which has a material nozzle, which distributes the material at the nozzle outlet, and air nozzles which guide the generated material stream of finely distributed droplets. According to the guidance of the air stream, round or elliptical coating surface areas arise, from which coating tracks are formed. The spray head cannot penetrate between high components because of its size. With the increasing distance which results from this, the spray jet broadens and generates splashes.
U.S. Pat. No. 6,170,760 B1 discloses an apparatus in which the air is routed in the form of a jacket around a coating material in the material nozzle. The lacquer is atomized when it leaves the spray valve. The nozzle may be of slender design, but cannot have an arbitrary length, because the pulsation effect belonging to this method is otherwise reinforced. Too slender a nozzle (<0.6 mm) also reinforces the pulsation and generates splashes. The pulsation arises because the coating material endeavors to assume the spherical shape which it assumes in the air stream as a function of the speed, nozzle diameter and viscosity of the coating material. The air which expands when it emerges from the nozzle generates splashes if the nozzle is too thin. Consequently, a narrow coating surface cannot be generated, but, instead, tracks with a width of only approximately 20 mm. Only low-viscosity lacquers can be processed. In the case of higher-viscosity materials, the pulsation increases, with the result that splashes occur, because both air and accumulations of substance are formed in the spray nozzle and lead to an irregular discharge of material.
U.S. Pat. No. 6,132,809 A discloses a rotatable double head which is equipped with a dispensing and a spray head.
Although the apparatus disclosed in DE 10261576 and mentioned initially has proved appropriate in practice, there still remains the wish to achieve improved application, higher flexibility and an improved coating of marginal contours with as low an outlay as possible in structural terms.